Hard initial state parton-parton collisions are responsible for the production of charm and heavier flavor and to direct virtual photon production through quark anti-quark annihilation via the Drell-Yan process.
Both processes give rise to hard virtual photon production dominating , depending on beam energy, different regions of the dilepton invariant mass spectrum.
It is commonly understood that these contributions can be calculated on the basis of perturbative QCD taking into account next to leading order graphs REF .
Perturbative methods, however, cannot straightforwardly be applied to beam energies below \(\sqrt{s_{NN}} \lesssim 10\)  G\(e\) V.
The calculation of charm production down to threshold energies might become possible, though, since long distance dynamics in the hard process described by so-called Sudakov corrections seem to dominate over the higher order corrections [1]}.
In contrast, the large energy density generated in the initial stage of the heavy-ion collision is by large due to multiple collision of soft partons, i.e. mostly gluons.
In this context it is interesting to note that pomeron exchange has recently found interest again to describe the non-perturbative (soft) interactions [2]}.
Pomerons can be understood as coherent multi-gluon states carrying the quantum numbers of the vacuum.
In particular they do not carry charge and consequently do not radiate (virtual) photons.
In the NEXUS model, the multi-parton collisions are separated into hard collisions (those appearing at large momentum transfer) and de-localized, soft collisions treated in an effective manner via string formation [3]}.
The stochastic treatment of the multi-parton collisions allows to study also the energy density variations over the initial fireball volume.
These distributions, which fluctuate event by event, are in particular responsible for the higher order angular modulations observed in the particle emission [4]}.
